Baclofen has a presynaptic action at the crayfish neuromuscular junction

The action of baclofen, a GABA analog, was studied at the crayfish neuromuscular junction (NMJ). Baclofen depressed the amplitude of excitatory junction potentials (ejps) without affecting muscle input resistance and reduced the frequency of spontaneous miniature ejps without affecting their size. Thus, baclofen may mediate presynaptic inhibition by depressing transmitter release from the excitatory nerve. The site of baclofen's effect at the crayfish NMJ may parallel its site of action in the vertebrate nervous system.     

The agrin/muscle-specific kinase pathway: new targets for autoimmune and genetic disorders at the neuromuscular junction.

The increasing understanding of the structural complexity of the neuromuscular junction (NMJ), and the processes that are important in its development, suggests many possible new disease targets. Here, we summarize briefly the genetic and autoimmune disorders that affect neuromuscular transmission, and the identified targets, including new evidence that antibodies to muscle-specific receptor tyrosine kinase (MuSK) are involved in the pathogenesis of acetylcholine receptor (AChR) antibody-negative myasthenia gravis. We then review the development of the NMJ, focusing on the important roles of nerve-derived agrin and MuSK in clustering of AChRs and other essential components of the NMJ.     

Myasthenia gravis: a retrospective study comparing thymectomy to conservative treatment

OBJECTIVES: To study the effectiveness of thymectomy (TY) in a group of patients with myasthenia gravis compared to a group of patients submitted to conservative treatment (CT) at a similar clinical stage. METHODS: Among 153 patients with myasthenia gravis, we paired 28 patients who underwent TY, with 28 cases under CT. The following data were analyzed: gender, age, and age at the beginning of symptoms, illness duration, follow-up time and type of medical treatment. There was no statistical difference between these 2 groups. The mean time for TY was 2.5 (0.2-13) years after the onset of the disease. The cases were evaluated through a functional scale at the beginning and at the end of the study. RESULTS: We found complete remission in 15 cases (TY 6, CT 9), improved (normal life with or without minimal symptoms and with or without medication) 9 cases (TY 8, CT 1), improved with partial control and minimal limitation 32 cases (TY 14, CT 18), and poor control 2 cases (TY 2). No death was found in this group. CONCLUSION: There was no statistical difference between the conservative treatment and thymectomy groups, regarding remission or improvement. Furthermore TY done in the first year of the disease or latter, did not change the final outcome.     

Developmental consequences of neuromuscular junctions with reduced presynaptic calcium channel function

Evoked neurotransmitter release at the Drosophila neuromuscular junction (NMJ) is regulated by the amount of calcium influx at the presynaptic nerve terminal, as for most chemical synapses. Calcium entry occurs via voltage-gated calcium channels. The temperature-sensitive Drosophila mutant, cac(TS2), has a reduced amount of calcium entry during evoked stimulation. We have used this mutation to examine homeostatic regulatory mechanisms during development of the NMJ on muscle 6 within the developing larva. The amplitude of the excitatory postsynaptic potentials are reduced for both the Ib and Is motor neurons in 3rd instar larvae which have been raised at 33 degrees C from the 1st instar stage. Larvae raised at 25 degrees C and larvae pulsed at 33 degrees C from the late 2nd instar for various lengths of time show a reduced synaptic efficacy as a 3rd instar. The results indicate that the nerve terminal cannot fully compensate physiologically in the regulation of synaptic transmission during larval life for a reduced amount of evoked calcium entry. Morphological comparisons of Ib and Is terminals in relation to length and numbers of varicosities are significantly reduced in cac(TS2), which also suggests a lack in homeostatic ability. These findings are relevant since many deficits in synaptic transmission in various systems are compensated for either physiologically or structural over development, but not in this case for reduced calcium entry during evoked transmission.     

Evaluation of neuromuscular transmission by using monopolar needle electrode

OBJECTIVE: To evaluate the value of single-fibre electromyography (SFEMG) with monopolar electrode (MNPE) in revealing neuromuscular transmission dysfunction. MATERIAL AND METHODS: We examined the extensor digitorum communis muscle by using single-fibre electrode (SFE) and MNPE sequentially, in randomly assigned 20 healthy volunteers and in 17 patients with known myasthenia gravis (MG). The high-pass filter setting was 3 kHz for MNPE. Ten individual jitter values were calculated for each electrode in every muscle. Repetitive nerve stimulation (RNS) test on trapezius muscle was performed on 15 patients. RESULTS: In controls, the mean jitter values were 27 +/- 9 (10-59) micro s with SFE, and 21 +/- 7.2 (9-56) micro s with MNPE (P = 0.001). In the MG group, the mean jitter values were 52.4 +/- 38 (12-221) micro s with SFE, and 51.8 +/- 34.7 (12-179) micro s with MNPE. Both electrodes identified junction dysfunction in 14 patients. RNS revealed decrement in four patients but 11. CONCLUSION: SFEMG with SFE is still the gold standard; however, SFEMG with MNPE is superior to RNS like SFEMG with SFE.     

Determining neuromuscular jitter using a monopolar electrode.

Neuromuscular jitter was determined in the extensor digitorum communis (EDC) of 41 healthy control subjects and 8 patients with myasthenia gravis (MG). Standard single-fiber electromyographic (SFEMG) techniques were used, except that a monopolar electrode (MPE) was substituted for a single-fiber electrode (SFE). In normals, mean jitter for 20 pairs was 22.4 +/- 2.8 microseconds and the mean jitter for an individual pair was 22 +/- 5.6 microseconds. In the age range tested (18 to 49 years), jitter did not change with increasing age. When MPE jitter studies were repeated in 16 normals, mean jitter usually varied by less than 10% (mean 7.4% +/- 6.2%) and remained normal. Jitter was significantly increased in MG compared to normals 73 +/- 56 microseconds versus 22 +/- 5.6 microseconds (F = 149.4, P less than 0.0001). Using an analog pain scale, the discomfort during electromyography using MPE and SFE was compared between the left and right arms in 35 normals. MPEs caused less discomfort than SFEs (P = 0.0031), and were preferred (71% versus 20%) to SFEs (chi 2 = 9.21, P = 0.01). Based on these results, we conclude that MPE determined jitter studies are reproducible, can distinguish between normals and MG, and are associated with less discomfort than SFE.     

Effects of napping on neuromuscular fatigue in myasthenia gravis

Introduction: The relationship between sleep and neuromuscular fatigue is understood poorly. The goal of this study was to evaluate the effects of napping on quantitative measures of neuromuscular fatigue in patients with myasthenia gravis (MG). Methods: Eight patients with mild to moderate MG were recruited. Patients underwent maintenance of wakefulness tests (MWT) and multiple sleep latency tests (MSLT). The Quantitative Myasthenia Gravis Score (QMGS) was measured before nap and after each nap to examine the effects of napping and sleep on neuromuscular weakness. Results: Results showed that QMGS improves only after naps where patients slept more than 5 min but not where patients did not sleep or slept less than 5 min. Conclusions: Daytime napping mitigates neuromuscular fatigue in patients with MG, especially if patients slept for more than 5 min. Muscle Nerve 48:816–818, 2013     

重症肌无力被动转移大鼠模型眼外肌的易感机制研究

目的探讨眼外肌在重症肌无力发病过程中的易感机制。方法给予SD大鼠腹腔注射mAb35建立重症肌无力被动转移(PTMG)大鼠模型,对照组大鼠注射等量生理盐水。选取PTMG组和对照组大鼠眼外肌、膈肌、胫前肌3种骨骼肌组织。采用乙酰胆碱酯酶(AChE)染色法观察神经肌肉接头(NMJ)并检测NMJ面积和灰度;采用银环蛇毒免疫组化法检测乙酰胆碱受体(AChR)数量;采用电镜观察NMJ超微结构和其AChR情况,并分析比较神经末端面积和突触后膜面积的比值以及突触前后膜长度的比值。结果 AChE染色结果显示,对照组眼外肌NMJ面积相对其他两种骨骼肌更小(P<0.01),PTMG组眼外肌与其他两种骨骼肌NMJ面积比较无统计学差异(P>0.05)。银环蛇毒免疫组化结果显示,PTMG组和对照组眼外肌与其它两种骨骼肌间AChR灰度值比较均有统计学差异(P<0.01)。电镜观察结果显示,PTMG组3种骨骼肌突触前后膜长度比值均较对照组下降(P<0.01),神经末端面积与突触后膜面积比值较对照组增加(P<0.01),其中眼外肌的变化较其他骨骼肌更为显著。结论 PTMG大鼠模型眼外肌易感机制可能与眼外肌和其他骨骼肌间NMJ面积、AChR数量差异造成眼外肌NMJ安全系数较低有关。     

Acute motor conduction block neuropathy or acute multifocal motor neuropathy: An attempt at a nosological systematization

Autoantibodies to muscle‐specific kinase (MuSK) can cause myasthenia gravis (MG). The pathophysiological mechanism remains unknown. We report in vitro electrophysiological and histological studies of the neuromuscular junction in a MuSK MG patient. Low levels of presynaptic acetylcholine release and small miniature endplate potentials were found. This combination of pre‐ and postsynaptic abnormalities was supported by histology, revealing partially denervated postsynaptic areas, and some degeneration of postsynaptic folds. Results suggest that anti‐MuSK antibodies reduce the stability of muscle–nerve contact. Muscle Nerve, 2010     

Glutamate receptors localize postsynaptically at neuromuscular junctions in mice

Dlg (Discs Large) is a multidomain protein that interacts with glutamate receptors and potassium channels at Drosophila neuromuscular junctions (NMJs) and at mammalian central nervous system synapses. Dlg also localizes postsynaptically at cholinergic mammalian NMJs. We show here that α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐proprionate (AMPA) receptor subunits, together with glutamate, are present at the mammalian NMJ. Both AMPA and NMDA (N‐methyl‐D‐aspartate) glutamate receptor subunits display overlapping postsynaptic localization patterns with Dlg at all NMJs examined in normal mice. Kir2 potassium channels also localize with Dlg and glutamate receptors at this synapse. Localization of the components of a glutamatergic system suggests novel mechanisms at mammalian neuromuscular synapses. Muscle Nerve 39: 343–349, 2009     

Passive transfer of seronegative myasthenia gravis to mice

Muscle weakness in myasthenia gravis is due to autoantibody-induced loss of functional acetylcholine receptors (AChR). About 15% of myasthenia gravis patients, however, do not have detectable anti-AChR antibodies. To investigate the effect of their plasma immunoglobulins on neuromuscular transmission, mice were injected with plasma (and in some cases purified immunoglobulin G (IgG)) from 7 “seronegative” myasthenia gravis (SMG) patients, and neuromuscular transmission parameters were examined. When injected for 15 days, all patients' plasma caused reductions in miniature endplate potential amplitudes, while endplate potential quantal content was significantly reduced by plasma from 4 of the 7 patients. There were no changes in ACh-induced depolarization or single channel properties, and ₁₂₅l-α-bungarotoxin binding studies showed no effect on AChR number, except in 1 case. Purified IgG injected for 3 days had similar effects to plasma injected for 15 days. Our findings confirm that SMG is autoantibody mediated and that there are pathogenic IgG antibodies. SMG appears to be a heterogeneous disorder and the target(s) for the antibodies may be diverse. © 1994 John Wiley & Sons, Inc.     

Minimal clinically important difference in myasthenia gravis: Outcomes from a randomized trial

Introduction: The minimal clinically important difference (MCID) is the smallest outcome change that has clinical significance. Its use has not been established in the study of myasthenia gravis (MG). Methods: Patients from a published intravenous immunoglobulin (IVIg) vs. placebo study were studied. One anchor‐based and 3 distribution‐based techniques were used to identify quantitative myasthenia gravis score (QMGS), repetitive nerve stimulation (RNS), and single‐fiber electromyography (SFEMG) MCID cut‐offs. Patients with a change‐score exceeding MCID cut‐offs were compared. Results: MCID cut‐offs were below a QMGS change of 3.0. Anchor‐based and 1 × SEM cut‐offs showed 58.3% vs. 30.7% responders (P = 0.017), ½ SD 54.2% vs. 19.2% responders (P = 0.018), and effect size 0.519 vs. 0.164 (P = 0.011) in IVIg vs. placebo. Anchor‐based (P = 0.73) and effect‐size (P = 0.41) MCID cut‐offs did not show a difference between IVIg and placebo. MCID methods did not produce meaningful RNS cut‐offs. Conclusions: QMGS MCID values provide clinically relevant information and are recommended in MG trials. MCID analysis shows that improvement in MG patients treated with IVIg reflects clinically meaningful changes. Muscle Nerve 49 : 661–665, 2014     

Guidelines for the treatment of autoimmune neuromuscular transmission disorders

Important progress has been made in our understanding of the cellular and molecular processes underlying the autoimmune neuromuscular transmission (NMT) disorders; myasthenia gravis (MG), Lambert–Eaton myasthenic syndrome (LEMS) and neuromyotonia (peripheral nerve hyperexcitability; Isaacs syndrome). To prepare consensus guidelines for the treatment of the autoimmune NMT disorders. References retrieved from MEDLINE, EMBASE and the Cochrane Library were considered and statements prepared and agreed on by disease experts and a patient representative. The proposed practical treatment guidelines are agreed upon by the Task Force: (i) Anticholinesterase drugs should be the first drug to be given in the management of MG (good practice point). (ii) Plasma exchange is recommended as a short‐term treatment in MG, especially in severe cases to induce remission and in preparation for surgery (level B recommendation). (iii) Intravenous immunoglobulin (IvIg) and plasma exchange are equally effective for the treatment of MG exacerbations (level A Recommendation). (iv) For patients with non‐thymomatous autoimmune MG, thymectomy (TE) is recommended as an option to increase the probability of remission or improvement (level B recommendation). (v) Once thymoma is diagnosed TE is indicated irrespective of the severity of MG (level A recommendation). (vi) Oral corticosteroids is a first choice drug when immunosuppressive drugs are necessary in MG (good practice point). (vii) In patients where long‐term immunosuppression is necessary, azathioprine is recommended together with steroids to allow tapering the steroids to the lowest possible dose whilst maintaining azathioprine (level A recommendation). (viii) 3,4‐diaminopyridine is recommended as symptomatic treatment and IvIg has a positive short‐term effect in LEMS (good practice point). (ix) All neuromyotonia patients should be treated symptomatically with an anti‐epileptic drug that reduces peripheral nerve hyperexcitability (good practice point). (x) Definitive management of paraneoplastic neuromyotonia and LEMS is treatment of the underlying tumour (good practice point). (xi) For immunosuppressive treatment of LEMS and NMT it is reasonable to adopt treatment procedures by analogy with MG (good practice point).     

Synaptic efficacy at singly- and dually-innervated neuromuscular junctions in the frog, rana pipiens

Electrophysiological investigation was performed on excised, curarized cutaneous pectoris and sartorius muscles of the frog. Sixteen percent of cutaneous pectoris and 7% of sartorius end-plates received dual innervation from two different axons. The resting membrane potential and input resistance of muscle fibers at single synapses were similar to those at dual junctions. The e.p.p.s. generated by individual axons in dual synapses were, on average, smaller in amplitude than e.p.p.s recorded from singly-innervated junctions. However, the total e.p.p. amplitude (sum of the two component e.p.p.s) at dually-innervated end-plates was similar to that of end-plates innervated by a single axon. Our results at polyneuronally-innervated end-plates are consistent with a model of simple competition between motor axons for a limited synaptic space.     

Facilitation of acetylcholine secretion at a mouse neuromuscular junction

Facilitation of transmitter secretion from motor nerve terminals following one or more conditioning stimuli was examined in mouse sternomastoid muscles. Following a single conditioning stimulus at 20 degrees C, facilitation decayed exponentially during the first 70-80 msec with a time constant of 60 msec. After 70--80 msec, a small slower component of facilitation was apparent. Initial facilitation (obtained by extrapolation back to 'zero' time) had a value of approximately 0.5. Following more than one conditioning stimulus (2-6), initial facilitation was greater but the pattern of decay was similar, the slower component becoming more obvious as the number of conditioning stimuli was increased. The slow decay phase also appeared exponential. The pattern of decay of facilitation could be well fitted by the sum of two exponentials, F1 (0)exp(--t/tau 1) + F2(0)exp(--t/tau 2). After a single stimulus at 20 degrees C, tau 1 and tau 2 had mean values of 35 and 163 msec. The main effect of increasing the number of conditioning stimuli was to increase (F1(0) and F2(0) with little change in tau 1 or tau 2. Changing temperature from 30 degrees C to 10 degrees C increased F1(0) and tau 2 but had relatively little effect on F2(0) and tau 1.     

重症肌无力的发病机制

重症肌无力(MG)是一种神经-肌肉接头传递功能障碍的自身免疫性疾病。MG的发病机制主要涉及免疫因素和遗传因素。免疫因素包括多种致病抗体、细胞免疫、细胞因子、补体及胸腺等。随着免疫学研究的不断进展,MG的发病机制也有了更深入的研究。